Multithreading-in-Java

[dzastin]

Hello,

I am looking at second example in Page 9 - Multithreading in Java

and i don't get expected output:
Put: 0
Get: 0
Put: 1
Get: 1
Put: 2
Get: 2
Put: 3
Get: 3
Put: 4
Get: 4

I get
run:
Put: 0
Get: 0
Put: 1
Get: 1
Put: 2
Get: 2
Get: 2
Get: 2
Put: 3 (Force STOP)

or

run:
Put: 0
Get: 0
Get: 0
Get: 0
Get: 0
Get: 0
Put: 1
BUILD STOPPED (total time: 2 seconds)

Maybe my neatbeans 7 beta has something to do with this?

class Queue {
   int exchangeValue;
    boolean busy = false;
    synchronized int get() {
       if (!busy)
          try {
             wait();
          } catch (InterruptedException e) {
               System.out.println(
                   "Get: InterruptedException");
          }
        System.out.println("Get: " + exchangeValue);
         notify();
       return exchangeValue;
    }
    synchronized void put (int exchangeValue) {
       if (busy)
          try {
             wait();
          } catch (InterruptedException e) {
               System.out.println(
                    "Put: InterruptedException");
          }
       this.exchangeValue = exchangeValue;
       busy = true;
       System.out.println("Put: " + exchangeValue);
       notify();
    }
}
class Publisher implements Runnable {
   Queue q;
   Publisher(Queue q) {
      this.q = q;
      new Thread (this, "Publisher").start();
   }
   public void run() {
      for (int i = 0; i < 5; i++){
         q.put(i);
      }
   }
}
class Consumer implements Runnable {
   Queue  q;
   Consumer (Queue  q) {
      this.q = q;
      new Thread (this, "Consumer").start();
   }
   public void run() {
      for (int i = 0; i < 5; i++){
         q.get();
      }
   }
}
class Demo {
   public static void main(String args []) {
     Queue q = new Queue ();
     new Publisher (q);
     new Consumer (q);
   }
}

[Petr]

Hi, This is code works fine. What you expected output?

[dzastin]

i get this:
Put: 0
Get: 0
Get: 0
Get: 0
Get: 0
Get: 0
Put: 1
BUILD STOPPED (total time: 3 seconds)

And i have to manually kill program. What IDE are you using? I will try uninstall 7 netbeans and install 6.9.1

[JosAH]

Your put() and get() methods aren't symmetrical: the busy variable should keep track whether or not a value has been put there (true); after it is read, the value should be false; change those two methods to:

    synchronized int get() {
       if (!busy)
          try {
             wait();
          } catch (InterruptedException e) {
               System.out.println("Get: InterruptedException");
          }
        int exchangeValue= this.exchangeValue;
        busy= false;
        System.out.println("Get: " + exchangeValue);
        notify();
        return exchangeValue;
    }
    
    synchronized void put (int exchangeValue) {
       if (busy)
          try {
             wait();
          } catch (InterruptedException e) {
               System.out.println("Put: InterruptedException");
          }
       this.exchangeValue = exchangeValue;
       busy = true;
       System.out.println("Put: " + exchangeValue);
       notify();
    }

kind regards,

Jos

[Petr]

Sometime in code is deadlock.
Deadlock (The Java™ Tutorials > Essential Classes > Concurrency)

[dzastin]

Thanks Josah,

everything works correctly now. Maybe the author of this example wanted to demonstrate one deadlock too :).

[JosAH]

Thanks Josah,

everything works correctly now. Maybe the author of this example wanted to demonstrate one deadlock too :).

If you have copied that example verbatim it was definitely incorrect. As you know now it has nothing to do with NetBeans; it just ran your (incorrect) code. It wasn't deadlock either, that variable was never set to false so the producer assumed that it had to wait (forever).

kind regards,

Jos